while the adaptations which the endoparasite requires are, in addition, (1) protection against the solvent action of the glands of its host, (2) the power of firm adhesion to a smooth internal surface, and (3) the ability not only to produce a large quantity of spermatozoa and ova, but in the absence of a fellow-parasite, to fertilise its own ova; and we find these conditions abundantly satisfied. {53}TREMATODA MONOGENEA (_ectoparasitica_). There are four subdivisions of the Monogenea:— I. _Temnocephalidae_, with four to twelve tentacles, and one sucker posteriorly (Fig. 20). II. _Tristomatidae_, with two lateral, anteriorly-placed suckers. Oral suckers are absent, a large posterior sucker is constant, and is often armed with hooks (Fig. 22, C). III. _Polystomatidae_, with, usually, two oral suckers and a posteriorly-placed adhesive disc armed with suckers and hooks (Figs. 23 and 24). IV. _Gyrodactylidae_ (Fig. 29). HABITS AND STRUCTURE OF ECTOPARASITIC TREMATODES. I. _Temnocephalidae._—These interesting forms, of which a good account has lately been written by Haswell,[71] occur on the surface (rarely in the branchial chamber) of fresh-water crayfish and crabs in Australasia, the Malay Archipelago, Madagascar, and Chili. Others have been found on the carapace of a fresh-water tortoise, and in the branchial chamber of the mollusc _Ampullaria_ from Brazil. Wood-Mason discovered others, again, in bottles containing spirit-specimens of Indian fish. _Temnocephala_ is rarely more than a quarter of an inch long, and looks like a minute Cephalopod or a broad flattened _Hydra_. By the ventral sucker each species adheres to its own particular host, the tentacles being used as an anterior sucker for "looping" movements. The food, consisting of Entomostraca, Rotifera, and Diatoms, is first swallowed whole by the large pharynx (Fig. 20, _ph_), which can be protruded through the ventrally-placed mouth, and is then received into a simple lobed intestine (_d_). The skin, especially on the surface of the tentacles, is provided here and there with patches of cilia borne by the cellular epidermis,—the only undoubted case of external cilia occurring in an adult Trematode. Minute rhabdites formed in special gland-cells, occur plentifully on the tentacles, and are another distinctly Turbellarian feature. The excretory system is peculiar (Fig. 21). Fine ducts proceed from the various organs of the body, and open to the exterior by means of a pair of contractile sacs {54}placed on the dorsal surface. Each sac is a single cell, and within it not one merely, but several "flames," or bunches of rhythmically contractile cilia, are present. These are placed on the course of excessively fine canals, which perforate the protoplasm of this cell. The terminal branches of the excretory canals end in branched cells, apparently devoid of "flames."